#### Riofunk7

##### Newbie level 4

I am looking for a fast double summation method for the following calculation.

X=[0:N-1]; Y=[0:N-1];

Z=ExEy[exp(-(X-Y)^2)];

Thus, in MATLAB,

X=[0:N-1]; Y=[0:N-1];

for n=1:N

Zy = sum(exp(-(X-Y))^2)) / N;

end

Z = sum(Zy) / N;

As shown above, firstly I tried "for loop" and "sum()" function in matlab. But for large number of sequence this method is too slow if the sample size N is large.

Are there anyone who know faster calculation for this problem?

Thank you in advance.

Rio.